1. Field of the Invention
The present invention relates to manufacturing methods of double-spiral arc tubes and to double-spiral arc tubes that are each formed by hanging and holding the substantially center of a softened glass on the top of a mandrel and winding the glass tube around the periphery of the mandrel.
2. Description of the Related Art
As an energy-saving era has started, low-pressure mercury discharge lamps that have high lamp efficiency and long lives, especially fluorescent lamps, have been attracting attentions. The inventors of the present invention have been studying the possibility of applying a glass tube bent into a double spiral to an arc tube used in such a fluorescent lamp. The advantageous effect of forming an arc tube into a double spiral is that this kind of arc tube has a longer discharge path and higher light-emission luminous flux than an arc tube that is of the equal size on the exterior, e.g. an arc tube including three U-shaped glass tubes that are joined together. The following describes manufacturing methods of a double-spiral arc tube. (For example, see the Japanese Unexamined Patent Application Publication No. 08-339780.)
FIGS. 1A and 1B show the process of bending a straight glass tube into a double spiral. FIG. 2 shows the positional relationship between a heating furnace and a mandrel.
As shown in FIG. 2, the heating furnace and the mandrel have a positional relationship by which the mandrel is disposed in front of the heating furnace so that the axis of the mandrel extends perpendicularly.
The following explains the process of forming a double spiral.
First, a glass tube 900 is heated with the use of a heating furnace 910, which could be an electric furnace. This heating process is performed, as shown in FIG. 1A, by placing an intermediate portion of the glass tube 900, which is to be formed into a double spiral, inside the heating furnace 910, and heating it until the intermediate portion gets soft.
When the intermediate portion of the glass tube 900 placed inside the heating surface 910 has become soft, the glass tube 900 is taken out of the heating furnace 910 by holding both ends thereof. The glass tube 900 is moved (in the direction indicated by the arrow) from the heating furnace 910 to above the mandrel 920 disposed in front of the heating furnace 910, like the glass tube 900a shown with the imaginary lines in FIG. 2. The glass tube is then positioned with the top 921 of the mandrel 920, like the glass tube 900b shown with the other imaginary lines in FIG. 2.
As shown in FIG. 2, the mandrel 920 has, at the top 921 thereof, a pair of hook units 923 and 924 by which the glass tube 900 is hung and held. As shown in FIG. 1B, the mandrel 920 also has, on the periphery thereof, winding grooves 922 along which the intermediate portion of the glass tube 900 is to be wound so as to be formed into a desired double spiral.
After the central portion 901b of the glass tube 900, shown in FIG. 2, is positioned between the pair of hook units 923 and 924, the mandrel 920 is rotated in the B2 direction as well as shifted (or moved) in the C direction, as shown in FIG. 1B.
As a result of the rotation of the mandrel 920, the glass tube 900 is hung and fixed on the hook units 923 and 924 positioned on the top 921 of the mandrel 920. Also as a result of the rotation and the shift of the mandrel 920, the glass tube 900 is wound along the winding grooves 922 on the mandrel 920.
It should be noted that the glass tube 900 is inflated during the winding process by nitrogen gas with a constant pressure being sent into the glass tube 900 from the ends, which is hung and held by the hook units 923 and 924 of the mandrel 920. With this arrangement, a portion of the outer surface of the glass tube 900 that is positioned on the winding grooves side is made abut on the wall surface of on the winding grooves 922 on the periphery of the mandrel 920, so that the cross sectional shape of the glass tube 900 conforms to the cross sectional shape of the winding grooves 922 of the mandrel 920.
When the glass tube 900 being wound around the mandrel 920 has made transition from the soft state to a hard state due to a temperature fall, the mandrel 920 is rotated in the reverse direction of the B2 direction, and the glass tube 900 being wound is removed from the mandrel 920. Thus, a glass tube 950 formed into a double spiral as shown in FIG. 1C is obtained.
Thereafter, various kinds of processing are applied to the obtained glass tube 950 according to methods that are publicly known. The processing includes: removal of the ends of the glass tube 950; application of a phosphor onto the inner surface of the glass tube 950; and enclosing and attaching electrodes at the ends of the glass tube 950. Thus, an arc tube will be obtained.
According to a conventional manufacturing method, however, there have been products with defects frequently because, for example, the glass tube 900 has not properly been wound along the winding grooves 922 of the mandrel 920, or even if the glass tube 900 has been properly wound, the double spiral happens to be deformed.
More specifically, there have been many cases where, when the glass tube 900 is wound around the mandrel 920, the glass tube 900 comes out of the winding grooves 922 of the mandrel 920, as shown in FIG. 3 (indicated with “954”).
In another case, as shown in FIG. 4, although the glass tube 950 is formed into a double spiral, the circumferential diameter of the double spiral gets large in some part (indicated with “951”), or the diameter of the glass tube gets small in some part (indicated with “952”, or, to the contrary, the diameter of the glass tube gets large in some part (indicated with “953”).
As a result of manufacturing arc tubes as trial mass-production by a conventional manufacturing method, the incidence ratio of having defective products as mentioned above during the process of forming straight glass tubes into double spirals was approximately 50 percent.